This invention relates to a vehicle brake squeal control device for suppressing so-called squeal phenomenon which develops from the brakes of a vehicle such as an automobile during braking.
So-called brake squeal that develops during braking by the brakes of a vehicle include relatively low-frequency sounds called groaning and irritating sounds in high-frequency bands of not less than several KHz (squeals). Various measures have heretofore been proposed to eliminate or reduce them. As one example, a vehicle brake device which has measures to prevent brake squeals is disclosed in JP patent publication 4-54325. The measures comprises a vibration detecting means for detecting vibrations by the piezoelectric effect of a piezoelectric element, and an oscillating means for damping the detected vibrations with the reverse piezoelectric effect of the piezoelectric element mounted on a friction member of the brake to prevent vibrations, thereby eliminating brake squeals.
Another brake control device is known from JP patent publication 11-278229. This control device comprises a control means for generating braking force corresponding to a detection signal from a sensor for detecting the stepping force of the brake pedal. While the brakes are being actuated, if the vehicle speed becomes below a predetermined value, the braking force of the brake device is changed at a predetermined cycle to suppress the resonance of various constituent members of the brake device.
Further, a brake noise preventive device of JP patent publication 2000-344072 is known. With this device, brake torque and vibrations of calipers are detected during braking while traveling at a speed below a predetermined value, and if they are above predetermined values and generation of noise is detected, a valve provided between the brake device and a reservoir is opened to reduce the brake pressure, or a valve provided between the brake device and a pressure accumulator is opened to increase the brake pressure, thereby preventing noise.
Another disk brake structure using brake squeal preventive measures disclosed in JP utility model publication 54-156079 is known. This disk brake is formed by bringing two pads held by a caliper into contact with a disk rotor by a brake cylinder, and shims are mounted on pad backing metals pressing two pads. The shims have a rubber member disposed between two steel plates.
By providing shims having a rubber coating on both sides of a steel plate provided between a brake pad and a mechanism for pressing it, excellent squeal preventive effect is obtained. But since the shims have poor durability, two steel plates are used to improve their durability and a rubber member is disposed therebetween to improve durability and ensure the squeal preventive effect by the shims.
Further, another brake hydraulic pressure control device is disclosed in JP patent publication 9-221013. It has a means for detecting brake squeals and a controller for changing the distribution ratio of brake hydraulic pressure to the front and rear wheels based on the squeal detection signals. If brake squeals are detected, control is made to reduce the distribution of the brake hydraulic pressure for the front wheels and increase it for the rear wheels.
Complicated factors are involved in brake squeal phenomena, and fundamental analysis about the causes of development of such squeals is needed. But in spite of various proposals made in these prior patent publications, no solution for fundamental factors has been made yet. About brake squeal phenomena, it is said that in a specific frequency band (7–9 KHz) squeals are large and rigid body vibration and bending vibration coexist. While they are irritating to the ear of the driver, such a noise is not always produced during braking. From experience, it is known that such a noise tends to be produced in the cold state or first in the morning.
But in any of the brake devices or control devices of JP patent publications 4-54325, 11-278229 and 2000-344072, consideration is scarcely given as to under what conditions brake squeal phenomena tend to occur. Thus, measures against prevention of squeals are not sufficient. For example, in JP patent publication 4-54325, vibrations are detected by a piezoelectric element and they are cancelled by the reverse piezoelectric effect of the piezoelectric element of the oscillating means. Thus, it is necessary to continuously actuate the device during travel to always detect vibrations irrespective of the conditions such as the temperature, braking force and number of revolutions when brake squeal phenomena occur.
Also, the detection ability of the piezoelectric element for sensing vibration is not uniform over the entire frequency range. Thus it will not respond in a specific frequency band. The brake control device of JP patent publication 11-278229 merely suppresses a resonance state only under conditions about the variation of the braking force. No consideration is given at all to other conditions such as temperature conditions and travel time. Also, simply by suppressing the resonance state, it is impossible to efficiently suppress squeal phenomena.
With the brake noise preventive device of JP patent publication 2000-344072, too, noise prevention is made only at caliper vibration above predetermined value, predetermined vehicle speed and brake pressure increasing or reducing. Thus, as with JP patent publication 11-278229, noise preventive measures are not sufficient. Also, brake squeals are produced not only during forward movement but during backward movement. There are none that have given consideration thereto.
Squeals are not always produced when braking is carried out. From experience, it is known that squeals normally tend to be produced in the cold of the calipers or after being left for a long time. But in the control devices disclosed in JP utility model publication 54-156079 and JP patent publication 9-221013, the fact that squeals tend to be produced in such a specific period is not considered at all. It is expected that the control devices of JP patent publications 54-156079 and 9-221013 can achieve the object of controlling squeals in a normal state (in which the temperature is stable) to a certain extent.
But the effect of suppressing squeals is small in the cold of the calipers or in a non-stable state after being left for a long time. This is because in the disk brake structure of JP patent publication 54-156079, only the durability of the shims have been improved, and simply by providing the shims, it is impossible to completely suppress squeals in a non-stable state. Also, in the control device of JP patent publication 9-221013, if squeals are detected, distribution of hydraulic pressure to the front and rear wheels is merely changed, so that the effect of suppressing squeals in the cold or after being left for a long time cannot be expected.
An object of this invention is to provide a brake squeal control device in which instead of carrying out squeal suppressing only with detection of the state which does not necessarily meet conditions of development of brake noise such as caliper vibration or a vehicle speed below a predetermined value, travel state and temperature conditions in which brake noise actually tends to be produced are specified to carry out squeal control. The brake squeal control device also provides control for suppressing brake noise during reversing.
The second object of the present invention is to provide a brake squeal control device in which the period or state in which brake squeals tend to occur most is specified, and in such a period or state, specific members of the brake means which can be the cause of brake squeals are actuated to build heat.